Use of specific versions by local ID number would not survive import/export across sites, making it very unsuitable for general usage. If something like this were considered it would probably need to be pretty explicitly unavailable in content pages.

Indeed; that is why I did not propose a more general identification.
The ids being used in permalinks to local pages that have not been
hidden are the only ones that (currently) are guaranteed to work.
They should luckily already cover a large class of pages that one
may want to be able to serve again with their unaltered historic
content, even if not with each of their old CSS.

If we want something more permanent, being constant across sites
and export/import, we need to use something like the message-id's
known from e-mail and netnews to identify revisions. Yet that would
have to be carefully considered and most likely existing revision
numbers converted which I am hesitant to propose.